Gears of war: When mechanical analog computers ruled the waves

In some ways, the Navy's latest computers fall short of the power of 1930s tech.

The Advanced Gun System, left, is intended to take on the role of the battleship's 16-inch guns, right. Aside from its GPS-guided shell, the digital technology of the AGS's fire control system does exactly what the USS Iowa's Rangekeeper Mark 8 did—just with fewer people and less weight.

US Navy

The USS Zumwalt, the latest destroyer now undergoing acceptance trials, comes with a new type of naval artillery: the Advanced Gun System (AGS). The automated AGS can fire 10 rocket-assisted, precision-guided projectiles per minute at targets over 100 miles away.

Those projectiles use GPS and inertial guidance to improve the gun’s accuracy to a 50 meter (164 feet) circle of probable error—meaning that half of its GPS-guided shells will fall within that distance from the target. But take away the fancy GPS shells, and the AGS and its digital fire control system are no more accurate than mechanical analog technology that is nearly a century old.

We're talking about electro-mechanical analog fire control computers like the Ford Instruments Mark 1A Fire Control Computer and Mark 8 Rangekeeper. These machines solved 20-plus variable calculus problems in real-time, constantly, long before digital computers got their sea legs. They were still in use when I served aboard the USS Iowa in the late 1980s.

There were a few efforts to marry these older systems to or replace them with digital technology during my tour, one of which (called the Advanced Gun Weapon System Technology Program) was remarkably like the AGS’s 100-mile shell: a GPS and inertially guided 11-inch dart-shaped shell wrapped in a 16-inch peel-away jacket, or sabot, that would have been able to fly nearly as far without the rocket assist thanks to the battleship’s big guns.

So why did the Navy never follow through with digitizing the battleship’s big guns? I asked retired Navy Captain David Boslaugh, former director of the Navy Tactical Embedded Computer Program Office, that question. And if anyone would know, it's Boslaugh. He played a role in the development of the Navy Tactical Data System—the forerunner to today’s Aegis systems, the mother of all digital sensor and fire control systems.

“At one time, my office was asked to do a study regarding upgrading the Iowa-class battleship fire control systems from analog to digital computers,” Boslaugh replied. “We found that digitizing the computer would improve neither the reliability nor the accuracy of the system and recommended, ‘Don't bother.’” Even without digital computers, the Iowa could fire 2,700-pound “dumb” shells nearly 30 miles inland with deadly accuracy, within a circle of probable error of around 80 meters. Some of its shells had circles of destruction larger than that.

Just how can a box of gears, cams, racks, and pins handle ballistics calculations based on differential equations with dozens of variables in real time? How does it manage to put a hunk of metal weighing as much as a Volkswagen Beetle on top of a target over the horizon in the first place? And how did this metal and grease out-calculate digital systems for so long? Let's start with a little bit of a history on battleship ballistics—complete with vintage Navy training films to show precisely how mechanical analog computing works.

Going ballistic

Shooting things with a gun from a ship is not exactly easy. In addition to the usual problems faced by ballistics—calculating how much bang to apply, how high to aim to reach a target at a certain range, how much to compensate for wind and the Coriolis effect—you have to take into account the fact that you’re shooting from a platform that has constantly changing pitch, yaw, and position. If you’re lucky enough to have a stationary target, the variables are still comparable to trying to hit something with a water balloon from the back of a hopping kangaroo.

Shooting things within sight of a ship is a feedback loop. Aim at the target, calculate its relative movement and other ballistic conditions, shoot, watch where the shot falls, and adjust. Shooting targets over the horizon is even trickier. It requires a forward observer who can give a precise geographic fix and then give corrections based on where shells land to walk them onto target.

In the days before turrets, ships fired guns in broadsides. Adjustments were generally made by where the shells fell and by waiting to fire until the side facing the enemy was on the upward side of a roll. But with the arrival of dreadnoughts and battle cruisers at the beginning of the 20th century, the range and lethality of ships’ guns both rose dramatically. There was now a greater need for accuracy, too.

That need corresponded with the rise of analog computers. Mechanical analog computers were used by astronomers for centuries to predict star positions, eclipses, and the phases of the moon—the earliest known mechanical analog computer, called the Antikythera Mechanism, dated to 100 BC. But nobody got around to using computers to try to kill people until much later.

Analog computers use a common set of mechanical devices to do their calculations—the same sorts of devices that convert the torque created by a car’s engine into turning wheels, lifting valves, and moving pistons. Data is “entered” into analog computers continuously, usually by the rotation of shaft inputs. A mathematical value is assigned to one full 360-degree rotation of the shaft.

In the days of the ancient Greeks, data entry was performed by turning a wheel. In more modern analog computers, variables from sensor data such as speed, direction, wind speed, and other factors were passed by electromechanical connections—synchro signals from gyrocompasses and gyroscopic “stable verticals,” tracking systems, and speed sensors. Constants, like passing time, were input by special constant-speed electrical motors.

Connecting all the shafts together to turn them into a continuous set of calculation outputs is a collection of gears, cams, racks, pins, and other mechanical elements that translate motion into math through geometric and trigonometric principles. This is also done through “hard-coded” functions that store the results of more complex calculations in their precisely machined shapes. Working together, these parts instantaneously calculate a very precise answer to a very specific set of questions: where will the target be when the giant bullet I push out of a 68-foot long rifled barrel gets there, and where do I need to aim to get it there?

When assembled precisely, analog computers can be much more accurate than digital computers on these types of questions. Because they use physical rather than digital inputs and outputs, they can represent curves and other geometric elements of calculations with an infinite level of resolution (though the precision of those calculations is based on how well their parts are machined, and loss from friction and slippage). There are no least significant digits dropped, and answers are continuous rather than dependent on “for-next” clock-driven computing cycles.

When assembled precisely, analog computers can be much more accurate than digital computers on these types of questions. Because they use physical rather than digital inputs and outputs, they can represent curves and other geometric elements of calculations with an infinite level of resolution. There are no least significant digits dropped, and answers are continuous rather than dependent on “for-next” clock-driven computing cycles.

Like yakumo said, the machining has to be impeccable on those parts. Well, the machining, and the calculations going into the design of the initial part.

possibly caused by spontaneous combustion of the gunpowder milled in the 1930s

If that is indeed what happened, I'm betting that the stabilizer was not doing its job anymore (all of it had reacted) and the nitrocellulose and/or other energetics in the gunpowder went rather unstable (assuming it was a NC based propellant).

I've heard that the Norden bomb sight, which also included an analogue computer, was a very highly guarded secret during World War II. Or, at least, as closely guarded as was practicable for a piece of technology that had to regularly fly over hostile territory to fulfil its function.

These analog computers are so awesome. As a kid (and a couple of times as an adult) we used t go to the USS North Carolina when we went to Wilmington (about a two hour drive). I remember seeing the fire control computers and they are quite large. When I first saw them I had no idea how complex they were, it was truly a magic black box to me. I've seen these videos before but it is really cool to learn more about them and think about the people tending them and giving read outs into the early 90s.

I loved this article. I've always been impressed by the analog computers on these ships, but I never came across videos explaining the mathematical concepts so well. Elegant in their simplicity. Kind of reminds me of building digital logic with discrete gates or relay logic.

Also, up until the 688 class submarines, fire control on nuclear submarines was all analog as well. As I recall there was a digital computing portion of the fire control system that had a massive 256 WORDS of storage. This was in the mid-70s. The advent of the 688's however, shifted to digital computation. Now submarines are massive computation engines using COTS (commercial off the shelf) systems that can be quickly (by military standards) enhanced.

possibly caused by spontaneous combustion of the gunpowder milled in the 1930s

If that is indeed what happened, I'm betting that the stabilizer was not doing its job anymore (all of it had reacted) and the nitrocellulose and/or other energetics in the gunpowder went rather unstable (assuming it was a NC based propellant).

From what I recall of the incident, age of the powder was less of a issue than improper storage temps where it was kept before being loaded onto the ship. Wherever the Navy was storing this powder the temps were well above what it should have been leading it to break down and it had become quite hazardous. There was also a considerable amount of needed maintenance that had not been completed. Combined together and it was a giant bomb waiting to happen.

Very cool background on the computers. Just goes to show how much human error can muck up things when you don't follow maintenance or storage protocols.

the machining on some of those old analogues must have been incredible.

Charles Babbage would have known a little something about that.

To be fair, Babbage was working toward mechanical digital computers, rather than analog ones. And, there were also electronic analog computers before digital electronics took over completely. But yeah, most of what Babbage spent his money on was improved machine tools, rather than actual computer making. The technology of his time simply wasn't physically capable of manufacturing his design in a useful way, so his real accomplishment was effectively more centered on making the first fab, rather than making the first computer.

Fantastic article but I was surprised to not see any mentions of the VT fuse. That was a misleadingly named radio proximity fuse used operationally in WWII. They were credited by Vannevar Bush, head of the U.S. Office of Scientific Research and Development during WWII for:

Stopping Kamikaze attacks.Destroying V-1 flying bombs when paired with radar controlled AA batteries.The Battle of the Bulge where the fuses detonated the shells just before they hit the ground.

They used 5" guns to fire at aircraft?!? I thought I had digested everything there was to know about these battlewagons, but I had no idea. I'm impressed.

Yes. The 5"/38 guns were used as heavy flak (and secondary surface armament), supplemented by the 40mm Bofors for medium range and 20mm Oerlikon for short range, at least during and immediately after World War II.

With the exception of the Doppler radar to measure the exit profile of each shell are both the analog and digital fire control systems coming up on the limits of the accuracy of the system? You can only make barrels of finite length after all. GPS and intertially-controlled ordinance is in its own way so cool. Trying building electronics and actuators that will withstand the thousands of g's acceleration in the barrel.

When assembled precisely, analog computers can be much more accurate than digital computers on these types of questions. Because they use physical rather than digital inputs and outputs, they can represent curves and other geometric elements of calculations with an infinite level of resolution. There are no least significant digits dropped, and answers are continuous rather than dependent on “for-next” clock-driven computing cycles.

Like yakumo said, the machining has to be impeccable on those parts. Well, the machining, and the calculations going into the design of the initial part.

The claim of "inifinite level of resolution" isn't accurate, you are limited to the precision at which you can machine the parts. Performing arbitrary precision calculations on a digital computer is also quite easy, but I guess that the performance was insufficient until recently.

Fascinating article. Going through 'A' and 'C' schools first as a DS and later as an FC, we were told about how analog fire control had come first, but the old-timers never gave any details about them.

As a point of correction, Aegis isn't a fire control system, it's an integrated computer suite. It has component computers that handle Weapons Control, SPY radar, Command and Decision, and Display systems. Each individual weapon system has it's own fire control computer taking inputs from various computers, radars, and operator inputs.

They used 5" guns to fire at aircraft?!? I thought I had digested everything there was to know about these battlewagons, but I had no idea. I'm impressed.

3" to 5" sized guns were common heavy anti-aircraft weapons in World War 2. The German 88 that armed the Tiger was originally an anti-aircraft gun, while the British had a variety of dual-purpose calibres (4 inch, 4.5 inch, 4.7 inch, 5.25 inch...)

One advantage was that destroyers with dual-purpose guns as their main armament were much more useful as fleet escorts in an anti-air role.

Modern naval autocannon are almost all universally dual-purpose weapons from the compact Bofors 57mm guns up to the 5 inch Mark 45.

Similar thing with fire control in tanks. In 1976 I joined a company where one of the products was a digital rendition of one of these analog computers, used for fire control in a tank (Chieftain IIRC) which could track and fire while on the move over uneven ground. The original design was analog electronic, but those were not linear and stable enough. The solution was to replace all the integrators, ratios, and differentiators by sequential-arithmetic digital circuits and then those circuits were patched together exactly as if they were analog (that was probably why they used sequential logic, to simplify patching). This kept everyone happier, with the "programming" so visible and literal, rather than converting it all to software (which at the time might have been tricky, the unit was probably more compact and lighter than any computer of the time).

I would guess tanks never previously had gun control. Mechanical units would have been too slow, and the early analog attempts were functionally sophisticated but unreliable. That tank was quite successful and videos of it rolling over undulating ground with the barrel staying on target were quite impressive.

Sean Gallagher / Sean is Ars Technica's IT Editor. A former Navy officer, systems administrator, and network systems integrator with 20 years of IT journalism experience, he lives and works in Baltimore, Maryland.